The pretreatment of cellulosic materials is an energy intensive process in which a liquid suspension of the cellulosic materials is pressurized and heated under controlled pH conditions. The pretreatment process enables glucans, xylans and other oligosaccharides to be removed from the cellulosic fiber and solubilzed within the liquid. The solubilized glucans, xylans, and other oligosaccharides may be enzymatically hydrolyzed to glucose and other monosaccharides in post processing steps and, thereafter, fermented to ethanol, amino acids, enzymes, and other bioproducts which are used as fuels or oxygenated chemicals that are growing in demand.
Economic feasibility of the process requires operating costs to be maintained at reasonable levels. Thus, the high energy associated with the pretreated cellulose must be recovered. Traditionally, heat exchangers have been employed to recover the high energy. However, heat exchangers, such as shell-and-tube heat, spiral heat exchangers, and plate and frame heat exchangers are relatively costly, raise several operating parameter concerns that must be controlled, and in some cases do not operate properly at high slurry concentrations. Cellulosic slurries on the cool side of such heat exchangers have not yet undergone pretreatment. As a result, their apparent viscosity may exceed 10,000 cp, which is difficult to pump. Furthermore, large pressure drops across the tubes and scaling in the tubes are other examples of common operating problems posed by shell-and-tube heat exchangers that must be controlled in order to achieve sufficient heat recovery. Moreover, the required high cellulosic slurry loading, defined as weight per unit volume, at the inlet of the heat exchanger, poses additional slurry handling problems conventional heat exchangers are not well-equipped to handle. Failure to control such operating parameters will lead to inefficient heat recovery and lower process yields. Inefficient heat recovery will make the energy costs of the process too high, thereby resulting in economic unfeasibility of the pretreatment process.
Heat exchangers also pose maintenance concerns. Heat exchangers contain heat transfer surfaces that foul, thereby requiring regular cleaning of the fouled surfaces. Such cleaning often consists of chemical cleaning systems. Additionally, the downtime produced from cleaning the heat exchangers necessitates expensive back-up heat exchange equipment, which can increase operating costs in a continuously operating plant.
Heat exchangers traditionally require a cooling working fluid, such as cooling water, on the shell side. The requirement for a cooling working fluid adds additional costs to the process. The cooling working fluid also does not effectively regenerate heat energy to other process fluids within the pretreatment process.
Thus, there is an unmet need for improved heat recovery from a high slurry stream in which the captured heat may be directly exchanged between an incoming and out-going slurry stream to thereby function as a recuperative heat exchange device. The improved heat recovery should efficiently handle the high slurry stream while being relatively inexpensive compared to other processing steps. The improved heat recovery also should be simple to operate, having minimal operating parameters to monitor and control.